TEHRAN — Iran lashed out against Western sanctions on Wednesday with new threats to cut oil sales to European countries and defiant claims of progress in its nuclear facilities, statements that U.S. officials dismissed as “bluster” and signs of increasing desperation within the country’s senior leadership.

On a day of confusing and sometimes contradictory claims, Iranian officials also signaled a willingness to negotiate with world powers over the future of Iran’s nuclear program. A letter from Iran’s senior nuclear negotiator to the European Union welcomed new talks as “the best means to broaden cooperation” and defuse the crisis, according to Iran’s semi-official Fars News Agency. E.U. officials confirmed the receipt of the letter.

The offer followed fiery speeches and TV appearances by President Mahmoud Ahmadinejad and Iran’s supreme leader, Ayatollah Ali Khamenei, who condemned the recent assassinations of Iranian scientists and promised retaliation against European countries that cooperated in a threatened oil embargo. Six E.U. countries were warned of a possible cutoff in Iranian oil deliveries, but hours later the country’s Oil Ministry appeared to back away from the threats.

The Obama administration dismissed the latest threats from Iran and said the apparent turmoil within the country’s senior leadership was further evidence that the West’s strategy of political and economic pressure is working.

“What we see is provocative acts, defiant acts, statements that are designed to distract attention from the demonstrated impact that the sanctions are having,” White House spokesman Jay Carney said. “We are very confident that the sanctions have put enormous pressure on the Iranian economy and on the Iranian regime.”

Other administration officials were scornful of what appeared to a choreographed effort by Iran to highlight advances by its nuclear scientists. Ahmadinejad appeared on Iranian television on Wednesday wearing a lab coat and a face mask to tout what he described as breakthroughs in the Iran’s nuclear energy program.

Among those claims was that Iran had started loading fuel rods into an aging U.S.-supplied nuclear reactor used to make medical isotopes. Separately, state-run Iranian television announced that Iran had begun operating a new generation of centrifuges at the country’s main uranium-enrichment facility at Natanz. Iranian officials have made similar claims of progress in the past.

In Washington, the State Department played down the nuclear accomplishments, describing them as “hyped” and “not terribly impressive.” Spokeswoman Victoria Nuland noted that Iran remains “many, many months behind” its own schedule.

A senior U.S. security official was similarly skeptical, noting that Iran appeared to have made only a modest step in its effort to create fuel rods for a civilian nuclear reactor.

“This in no way enhances Iran’s ability to make nuclear weapons,” said the official, who spoke on the condition of anonymity in order to discuss confidential intelligence assessments of Iran’s nuclear progress.

Still, the boasts of nuclear achievements appeared likely to add to mounting tensions between Iran and the United States, Israel and many Western nations over concerns that Iran is intent on developing nuclear weapons, even though Iranian officials insist their program is solely for peaceful energy production.

The war of words between Iran and Israel intensified this week after an apparent assassination attempt on Israeli diplomats in New Delhi and the former Soviet republic of Georgia. Israel also alleged that Iran was behind a series of explosions in Thailand.

Iran has denied responsibility in the attacks and blames Israel for the assassinations of several scientists who have worked in Iran’s nuclear program over the years.

Iran’s threats to cut oil exports to European nations appeared aimed at upstaging the European boycott of Iranian oil, which was approved by E.U. ministers last month and is is scheduled to start in July.

Europeans are also mulling the further step of excluding key Iranian banks from the global electronic banking system known as SWIFT, the Society for Worldwide Interbank Financial Telecommunications. Expulsion from the Belgium-based network would effectively end the banks’ ability to conduct business internationally.

Early on Wednesday, Iran’s official Press TV said Iran would stop exporting oil to six E.U. countries — France, the Netherlands, Greece, Italy, Spain and Portugal. The announcement helped drive the price of crude to nearly $102 a barrel. . Europe accounts for about 18 percent of Iran’s crude exports, with Greece, Italy and Spain among the main buyers, according to the Associated Press.

Iran’s Oil Ministry subsequently denied the Press TV report. Fars News Agency, meanwhile, quoted an Oil Ministry source as saying that the exports to Europe have not been stopped yet but that Iran has given an ultimatum to those countries to continue their long-term contracts. Iran’s Arabic-language state television channel al-Alam said the ministry would provide more details Thursday.

The technological developments announced by Iran had been previously alluded to by nuclear officials, though with less fanfare than was on display during Wednesday’s broadcast. State television said each of the projects would adhere to Iran’s nuclear slogan: “nuclear energy for all, nuclear weapons for none.”

In an emotional speech during the ceremony, the mother of Mostafa Ahmadi-Roshan, a deputy head of the Natanz uranium-enrichment facility who was killed last month, asked her son’s colleagues to follow his path and continue their groundbreaking research.

Ahmadinejad stressed that progress requires struggle against world powers, which he asserted are much weaker than they seem. “If today any nation wants to taste dignity, progress, justice, freedom and greatness, it has to break through the imposed policies of the tyrants,” he said.

Israel and its Western allies charge that the moving of centrifuges to a mountain site said to be impregnable to bunker-busting bombs is a sign that Iran is trying to hide parts of its nuclear program.

The International Atomic Energy Agency — the U.N. watchdog that monitors all known Iranian nuclear activities, including those at the Fordow bunker site — confirmed last month that Iran had started enriching uranium there.

High-ranking officials from the IAEA are scheduled to visit Iran on Tuesday for talks, possibly signaling that Iran is ready to provide more transparency on the goals of its nuclear program. Such transparency is a key demand by the United Nations in recent resolutions against Iran.

Warrick reported from Boston; staff writer William Branigin in Washington contributed to this report.

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